In certain control aparatus, such as, in pressure monitoring systems for steel making furnaces, power generating plants, etc., it is necessary to closely control the exhaust gas pressure in the smokestack. Normally, the furnace smokestacks are provided with butterfly types of control valves which are power operated to manipulate the opening of the smokestack in order to continuously regulate the exhaust gas pressure. Presently, the butterfly valves are controlled by hydraulic types of servo positioning cylinders that are situated within the confines of the furnace smokestacks. Such an enclosed hydraulic system is not completely satisfactory since it is costly to initially purchase and expensive to subsequently maintain. Further, there was difficulty in replacing and repairing a malfunctioning servo system due to its remote location in the smokestack. Further, the extremely dirty and relatively high temperature conditions existing in smokestacks resulted in premature failure and caused unreliability of the servo types of control valves. In addition, it is found impractical and uneconomical to provide hydraulic conduits or lines leading from the smokestack to a central control center due to problems of leakage and the need of extremely high pressures. Thus, a viable exhaust pressure control system for smokestacks requires a minimum amount of components or parts within the smokestack to increase reliability and serviceability of the apparatus.